A golden age ensued during the sixteenth century after the Union of Lublin which gave birth to the Polish–Lithuanian Commonwealth. The szlachta (nobility) of Poland, far more numerous than in Western European countries, took pride in their freedoms and parliamentary system. For 10 years between 1619 and 1629 the Polish-Lithuanian Commonwealth was at its greatest geographical extent in history, incorporating most of what today is Poland, Ukraine, Belarus, Lithuania, Latvia, Estonia, and some parts of modern Russia. The period starts in 1619 when the Russo-Polish Truce of Deulino came into effect, whereby Russia conceded Commonwealth control over Smolensk and several other border territories. In 1629 the Swedish-Polish Truce of Altmark came into effect, whereby the Commonwealth conceded Swedish control over most of Livonia, which the Swedes had invaded in 1626.

In the mid-seventeenth century, a Swedish invasion ("The Deluge") and the Cossacks' Chmielnicki Uprising which ravaged the country marked the end of the golden age. Famines and epidemics followed hostilities, and the population dropped from roughly 11 to 7 million